Ready-prepared meals which are enjoying increased growth rates in Europe are transferred to trays after their preparation (cf. FIG. 1). A film which is heat sealed to the edge of the tray seals the packaging and protects the ready-prepared meal from external influences. The ready-prepared meals are suitable, for example, for heating in a microwave and in a conventional oven. The ready meal and the packaging have to be “dual ovenable” (=suitable for microwave and conventional ovens). As a consequence of the temperatures existing in a conventional oven (up to 220° C.), particularly high demands are made on the packaging material (tray and lid film).
Typical materials, suitable for microwave and conventional ovens, for the tray and the lid film are (PET=polyethylene terephthalate, CPET=crystalline PET, APET=amorphous PET)
Tray: CPET, aluminum, cardboard coated with PET or with PET film or trays made of APET/CPET. Trays made of APET/CPET (cf. FIG. 1) include externally a CPET layer and internally, i.e. facing toward the ready-prepared meal, an APET layer. The thick, crystalline CPET layer provides the stability of the tray, even at the comparatively high temperatures in a conventional oven. The amorphous PET essentially improves the adhesion of the film to the tray.
Lid film: here, PET is generally used which remains dimensionally stable and solid enough even at 220° C. Materials such as PP and PE are ruled out owing to their low melting points. The demands on the lid film are best fulfilled by biaxially oriented polyester films.
When preparing the ready-prepared meal in an oven, the polyester film is removed by hand from the tray shortly before heating or shortly after heating. When this is done, the polyester film must on no account start to tear, start and continue to tear or tear off. The removal of the film from the tray without the film starting or continuing to tear or tearing off is also referred to in the foods industry as peeling. For this application, the polyester film therefore has to be not only heatsealable, but in particular also peelable. For a given material and given overall thickness of the film, the peelability of the film is determined mainly by the properties of the surface layer of the film which is sealed to the tray.
The peelability of films can be determined relatively simply in the laboratory using a tensile strain tester (for example from Zwick, Germany) (cf. FIG. 2). For this test, two strips of width 15 mm and length approx. 50 mm are cut out of the polyester film and the tray and sealed to one another. The sealed strips are, as shown in FIG. 2, clamped into the clips of the tester. The “angle” between the film clamped in the upper clip and the tray strip is 180°. In this test, the clips of the tester are moved-apart at a speed of 200 mm/min, and in the most favorable case the film is fully peeled off from the tray (cf., for example, ASTM-D 3330).
In this test, a distinction is to be drawn between essentially two different mechanisms.
In the first case, the tensile force rises rapidly in the course of the pulling procedure up to a maximum (cf. FIG. 3a) and then falls directly back to zero. When the maximum force is attained, the film starts to tear or, before delamination from the tray, tears off, which results in the force falling immediately back to zero. The film is in this case not peelable, since it is destroyed. The behavior of the film can rather be described as a kind of “welding” to the tray. The destruction of the film on removal from the tray is undesired, because this complicates the easy opening of the packaging without tools such as scissors or knives.
In contrast, a peelable film is obtained when the tensile force or the peeling force rises up to a certain value (i.e. up to a certain plateau) and then remains approximately constant over the distance over which the two strips are sealed together (cf. FIG. 3b). In this case, the film does not start to tear, but rather can be peeled as desired off the tray with a low force input.
The size of the peeling force is determined primarily by the polymers used in the outer layer (A) (cf. FIG. 4, polymer 1 and polymer 2). In addition, the size of the peeling force is dependent in particular on the heatsealing temperature employed. The peeling force generally rises with the heatsealing temperature. With increasing heatsealing temperature, the risk increases that the sealing layer might lose its peelability. In other words, a film which is peelable when a low heatsealing temperature is employed loses this property when a sufficiently high heatsealing temperature is employed. This behavior is to be expected in particular in the case of polymers which exhibit the characteristics shown in FIG. 4 for polymer 1. This behavior which tends to generally occur but is rather unfavorable for the application has to be taken into account when designing the sealing layer. It has to be possible to heatseal the film in a sufficiently large temperature range without the desired peelability being lost (cf. polymer 2 in FIG. 4). In practice, this temperature range is generally from 150 to 220° C., preferably from 150 to 200° C. and more preferably from 150 to 190° C.
The heatsealable and peelable layer is applied to the polyester film in accordance with the prior art, generally by means of offline methods (i.e. in an additional process step following the film production). This method initially produces a “standard polyester film” by a customary process. The polyester film produced in this way is then coated offline in a further processing step in a coating unit with a heatsealable and peelable layer. In this process, the heatsealable and peelable polymer is initially dissolved in an organic solvent. The final solution is then applied to the film by a suitable application process (knifecoater, patterned roller, die). In a downstream drying oven, the solvent is evaporated and the peelable polymer remains on the film as a solid layer.
Such an offline application of the sealing layer is comparatively expensive for several reasons. First, the film has to be coated in a separate step in a special apparatus. Second, the evaporated solvent has to be condensed again and recycled, in order thus to minimize pollution of the environment via the waste air. Third, complicated control is required to ensure that the residual solvent content in the coating is very low. Moreover, in an economic process, the solvent can never be completely removed from the coating during the drying, in particular because the drying procedure cannot be of unlimited duration. Traces of the solvent remaining in the coating subsequently migrate via the film disposed on the tray into the foods where they can distort the taste or even damage the health of the consumer.
Various peelable, heatsealable polyester films which have been produced offline are offered on the market. The polyester films differ in their structure and in the composition of the outer layer (A). Depending on their (peeling) properties, they have different applications. It is customary, for example, to divide the films from the application viewpoint into films having easy peelability (easy peel), having moderate peelability (medium peel) and having strong, robust peelability (strong peel). The essential quantifiable distinguishing feature between these films is the size of the particular peeling force according to FIG. 3b. A division is undertaken at this point as follows:
Easy peelabilityPeeling force in the range(easy peel)from about 1 to 4 N per 15 mm ofstrip widthModerate peelabilityPeeling force in the range(medium peel)from about 3 to 8 N per 15 mmof strip widthStrong, robust peelabilityPeeling force in the(strong peel)range of more than 5 N per15 mm of strip width
Sealable PET films and processes for their production are known.
EP-A 0 379 190 describes a biaxially oriented, multilayer polyester film comprising a carrier layer of polyester and at least one sealing layer of a polyester composition. The polyester film cam be produced by employing coextrusion technology, inline coating, inline lamination or employing suitable combinations of the technologies mentioned. In inline coating, the polymers of the sealing layer are applied to the carrier layer in the form of a dispersion or solution. In inline lamination, the polymers of the sealing layer are applied to the carrier layer in the form of extruded melt, for example between the two stretching steps.
The sealing layer may comprise aliphatic and aromatic dicarboxylic acids and also aliphatic diols. The polymer for the sealing layer comprises two different polyesters A and B, of which at least one (polyester B) contains aliphatic dicarboxylic acids and/or aliphatic diols. The sealing energy which is measured between two sealing film layers facing each other and bonded together (=FIN sealing) is more than 400 gforce*cm/15 mm (=more than 4 N*cm/15 mm), and the sealing film layer may comprise inorganic and/or organic fine particles which are insoluble in the polyester, in which case the fine particles are present in an amount of from 0.1 to 5% by weight, based on the total weight of the sealing film layer. Although the film features good peeling properties (having plateau character in the peeling diagram, see above) with respect to itself (i.e. sealing layer with respect to sealing layer), there is no information about the peeling performance with respect to trays made of APET, CPET and APET/CPET. In particular, the film of this invention is in need of improvement in its producibility and its processibility.
WO A-96/19333 describes a process for producing peelable films, in which the heatsealable, peelable layer is applied inline to the polyester film. In the process, comparatively small amounts of organic solvents are used. The heatsealable, peelable layer comprises a copolyester which has a) from 40 to 90 mol % of an aromatic dicarboxylic acid, b) from 10 to 60 mol % of an aliphatic dicarboxylic acid, c) from 0.1 to 10 mol % of a dicarboxylic acid containing a free acid group or a salt thereof, d) from 40 to 90 mol % of a glycol containing from 2 to 12 carbon atoms and e) from 10 to 60 mol % of a polyalkyldiol. The coating is applied to the film from an aqueous dispersion or a solution which contains up to 10% by weight of organic solvent. The process is restricted with regard to the polymers which can be used and the layer thicknesses which can be achieved for the heatsealable, peelable layer. The maximum achievable layer thickness is specified as 0.5 μm. The maximum seal seam strength is low, and is from 500 to 600 g/25 mm2, or [(from 500 to 600)/170] N/15 mm of film width.
WO 02/059186 A1 describes a process for producing peelable films, in which the heatsealable, peelable layer is applied inline to the polyester film. The method employed is melt-coating, and it is preferably the longitudinally stretched film which is coated with the heatsealable, peelable polymer. The heatsealable polymer contains polyesters based on aromatic and aliphatic acids, and also based on aliphatic diols. The copolymers disclosed in the examples have glass transition temperatures of below −10° C.; such copolyesters are too soft, which is why they cannot be oriented in customary roll stretching methods (adhesion to the rolls). In WO 02/059186 A1, the melt-coating known per se is delimited from the extrusion coating known per se technically and by the viscosity of the melt. A disadvantage of the melt-coating is that only comparatively fluid polymers (max. 50 Pa·s) having a low molecular weight can be used. This results in disadvantageous peeling properties of the film. Moreover, the coating rate in this process is limited, which makes the production process uneconomic. With regard to quality, faults are observed in the appearance of the film which are visible, for example, as coating streaks. In this process, it is also difficult to obtain a uniform thickness of the sealing layer over the web width of the film, which in turn leads to nonuniform peeling characteristics.